Blizzard announced that for a limited time players who make a 12-month subscription commitment to World of Warcraft through the WoW Annual Pass will receive Diablo III for free. The game will be made available via Battle.net for digital download. Other incentives include a Tyrael’s Charger flying mount in patch 4.3 for all future and current characters on your account as well as, guaranteed access to the beta test for WoW’s next expansion, Mists of Pandaria.
Keep in mind however, according to their FAQ if you go for the Annual Pass, you will not be able to cancel your subscription until the 12 month subscription has ended and if your subscription lapses you lose all of the incentives.
Source: Blizzard Blog